Degassing aluminum articles



aluminum and aluminum base alloy articles.

United States Patent 2,995,478 DEGASSING ALUMINUM ARTICLES Fred Keller,New Kensington, and Edmund C. Franz, Pittsburgh, Pa., assignors toAluminum Company of America, Pittsburgh, Pa., a corporation ofPennsylvania No Drawing. Filed Feb. 17, 1959, Ser. No. 793,705 8 Claims.(Cl. 14811.5)

This invention relates to a method for the extraction of gas and theelimination of voids and flakes in wrought The term aluminum will beused herein to encompass aluminum and aluminum base alloys which containat least 50 percent by weight of aluminum.

Fnished and semi-finished aluminum articles occasionally containoccluded gas, principally hydrogen, which may give rise to objectionablediscontinuities in the metal structure. Some of the hydrogen is usuallyconsidered to be in solution in the solid metal, i.e. it is in themonatomic state, although pockets or voids filled with molecularhydrogen have also been observed. In the fabrication of wrought articlesfrom the ingot, some thermal treatments are generally employed to aid inworking the metal or to develop the desired strength, and it isconsidered that such heating also produces diffusion of the monatomichydrogen to any voids or discontinuities within the metal whereatassociation into molecular form takes place. The problem of so-calledflakes" within the internal metal structure has been traced to thesehydrogenfilled voids.

Because of the gas pressures developed by the molecular gas, subsequentworking of the metal does not effect a healing of the void ordiscontinuity, and heating of the article at elevated temperatures mayincrease such pressures to the point where the metal sutfers localplastic deformation.

The problem of occluded gas has become increasingly important with thegrowing requirements for high strength aluminum articles. Any gas-filledvoid may not only constitute an area of weakness in the final article,but may give rise to flakes, blisters, slivers and other defects whichresult in rejection. These problems have prompted investigations to finda method for the elimination of occluded gas and voids associatedtherewith.

It has heretofore been proposed that hydrogen gas contained in aluminumarticles may be driven out of the metal by heating under a vacuum attemperatures on the order of 500-1000 F. Commercial utilization of thisprocedure has not proven feasible and attempts to remove gas in anundried air'atmosphere have been unsuccessful. Also, it has beensuspected that the degassed articles are prone to again absorb gas.

It has nowbeen discovered that an aluminum article having a low gascontent, or substantially none, can be produced by a method in which analuminum body containing gas and voids is coated with at least one metalselected from the group consisting of nickel and chromium and heated ina gaseous atmosphere at a temperature above 750 F., but below that whichany substantial amount of fusion occurs, for a length of time sufficientto ditfuse occluded gas into the atmosphere surrounding the article.

The metal article may be thereafter worked to plastically deform themetal article. The metal coating is also highly beneficial in preventingoxidation and blistering of the aluminum during subsequent heating operations.

To bring about the extraction of gas from the aluminum article, theheating step must be conducted under conditions which facilitate gasremoval. It has been found that this can be accomplished by initiallycoating the article with nickel or chromium or both metals to form afilm on the metal surface which reduces greatly, if it does notaltogether eliminate, the existence of monatomic hydrogen at the metalsurface, as well as tending to inhibit oxidation of the aluminum. Thenature of the mechanism by which the gas is driven out of the aluminumarticle is not fully understood; however, it seems to involve anirreversible conversion of monatomic hydrogen into molecular hydrogen.By employing the metal film on the article, the degassing or prolongedheating step may be carried out in a normal atmosphere without danger ofre-absorbing gas, thus removing one of the great economic handicaps tothe use of long-time heating procedures to extract gas from aluminumarticles.

Prior to coating the surface of an aluminum article with nickel orchromium, or both, the surface of the article should be treated toremove any oxide film. This may be conveniently accomplished by dippingor otherwise applying at 1% aqueous solution or hydrofluoric acid to thearticle. Other suitable solutions can be used, of course. The nickel andchromium coatings may be deposited on the surface in any convenientmanner. Electro-plating and electro-less plating techniques have beenhighly successful, as have been metallic spray coatings. The thickne'ssof the coating is not critical, and apparently it need not becontinuous, but it should be relatively thin; such as normally producedby plating or spraying procedures. For example, coatings of less than0.1 mil have been satisfactory. Furthermore, a coating initially appliedto plate has been found to protect it through the ensuing reductionsteps to sheet.

The electrolytic coatings may be applied directly or they may be appliedover a base coating or strike of another metal, such as copper, inaccordance with conventional practice. The copper strike has no effecton the treatment of the present invention. If desired, a duplex coatingof nickel and chromium may be employed wherein an initial layer of onemetal is deposited and the second one applied to the first coating. Itis also possible to mingle the two metals, nickel and chromium, as inapplying a sprayed coating.

The coating and degassing steps may be conducted at any step of theworking operation. Coatings applied to plate slabs have been found toprovide protection throughout the rolling sequence and its variouspreheating operations. For this reason, it is often desirable to leavethe protective metal coating on the aluminum article until after thefinal heat treatment.

The coating may be removed by any convenient method. Generally, it ismost convenient to strip the metal electrolytically in a sulfuric acidelectrolyte.

After the metal coating has been applied, the article may be degassed ina conventional air atmosphere furnace. No drying of the air need beundertaken as moisture can be tolerated in the gas extraction step, thuspermitting employment of conventional industrial furnace atmosphereswhich generally contain 1.5 to 30 grains of water per cubic foot. Gaseswhich are inert or non-deleterious to aluminum may be employed in placeof air such as nitrogen, argon, helium and fuel gas, or such gases maybe used in admixture with air.

The term atmosphere, as used herein, includes air, gases inert toaluminum, or combinations thereof, and moisture associated with air andother gases.

The duration of the heating step will be dependent upon the thickness ofthe article being treated (the shortest diffusing path), the desiredfinal gas content of the metal and the temperature employed. The rate ofditfusion increases almost exponentially with increase in temperature.Since commercial degassing of large quantities of aluminum articlesrequires space-consuming heating equipment, it is. desirable that theheating step be of as short duration as possible. Therefore, atemperature at least above 750 F., and generally above 900 F., should beused. The temperature is preferably below the temperature of incipientfusion, but temperatures above the melting point of one or more of thephases have been successfully employed where the amount is very smalland eutectic melting has not been a concern. However, the article shouldnot be heated at temperatures which adversely affect the properties ofthe metal. When the gas-containing metal is heated in this manner, themajor portion of the gas is driven off within a reasonably short time, aproportionately longer time being required to remove the last fewpercent of gas. For purposes of this application, an article will beconsidered substantially degassed or gas-free if the gas has beensubstantially diffused out of the internal discontinuities to permitsubsequent healing, although some may remain in solution in the metal.Generally, this will require removal of at least 75 percent or more ofthe occluded gas, although it may often be desirable to extract as muchas 90 percent, or more.

Theoretically, the length of time for degassing increases as the squareof the half-thickness of the metal body. Therefore, in some cases, itmay be desirable only to seek extraction of the gas from relatively thincross-sections of the articles where the strength characteristics are ofprimary concern rather than to degas the entire article which mightrequire a much longer time.

Indicative of the variables governing the diffusion step, Tables I andII are a guide to the time theoretically necessary at severaltemperatures for removing various percentages of gas, as based on Fickslaw and the diffusion constant for hydrogen in aluminum. These tablesgive a time factor per centimeter half-thickness (or radius) which maybe converted to the ideal length of time necessary to degas a giventhickness of metal by multiplying the factor by the square of thehalf-thickness of the metal body in centimeters.

d 2 T X where:

T=time necessary for degassing article (in hours) t=time factor for unitthickness (from table) d=thickness (or diameter) of article (incentimeters) TABLE I [Time factor for sheet, plate, or rectangularcross-section, hrs/unit centimeter half-thickness] 'Iemp., C. PercentRemoval TABLE II [Time factor (or rod or bar, hrs/unit centimeterradius] Temp., C. Percent Removal For most aluminum articles, 850-1000F. (450-540 C.) is a temperature range conveniently employed. Inpracticing the invention at a temperature of 940 F. (505 C.), sincecommercial conditions are far from ideal, a rule of thumb figure hasbeen to maintain aluminum forgings at temperature at least 16 andpreferably 24 hours or more per inch of thickness for adequate gasremoval. However, occasionally articles having a thickness of overseveral inches require shorter times but often require more than 24hours per inch of thickness.

In the treatment of rolled articles at the same temperature, at leastabout 4 hours and preferably six hours are used for a half-inchthickness. However, thinner sheet products degas very quickly, .091 inchthick sheet has been degassed at 940 F. in only 1 minute. Because of thedifiiculty in removing gas from some articles, it is conceivable thatthe rate may vary with the mode of fabrication or grain orientation orwith the surface condition. For this reason and '(also for obtaining amore definite determination of the time necessary to degas a particulararticle, the testing of samples is desirable for the establishment ofconditions for the heating step. Similarly, the time necessary'fordegassing compressed aluminum powder products will vary with theconditions under which the compact was prepared.

Subsequent to the heating step, the article is subjected to a workingoperation for effecting plastic deformation of the metal and to healvoids left by the diffused hydrogen. The various working methods may beemployed singly or in combination to effect the welding of the voids.The term forging includes both hammer-forging and press-forging methods.The amount of working or percentage of reduction necessary will bedependent upon the nature of the article and the original content ofvoids. In some cases, especially in larger articles such as dieforgings, a relatively small reduction may be suficient to heal or weldthe discontinuities in the structure. Generally, in die forgings areduction of from to 50 precent by a blocking or finishing operation hasbeen found to be satisfactory, although even greater reductions mayoccasionally be necessary; hand forgings may necessitate reductions of 2to 50 percent. Although extrusion operations will generally healdiscontinuities, it is frequently desirable to first forge the metalbillets to a reduction in thickness of 2 to 50 percent. Similarly, apreliminary forging is sometimes desirable before a rolling operation.

The degassed and healed aluminum articles may then be subjected tofurther heat treatments. Because the voids or discontinuities within themetal structure no longer exist, the problem of gassing (or regassing)is minimized unless new discontinuities are subsequently created withinthe metal structure.

The problem of gaseous occlusions is most pronounced in the case ofaluminum base alloys containing magnesium and/or zinc. alloys as well asaluminum itself may often require degassing dependent upon theconditions to which the A lot of commercial purity aluminum (99.00percent minimum) plate panels, 6" x 9", of two different thicknesses,0.95 and 0.35 inch, was divided into two groups. The first group waschemically plated with nickel to a thickness of 0.1 mil whereas thesecond group was left bare. Both were then heated at 940 F. in a conven-24 hours and the 0.35 inch material for 3 hours. 'The However, otheraluminum base nickel coating was then electrolytically stripped in asulfuric acid bath. The two plate sizes were then rolled to a thicknessof 0.35 and 0.25 inch, respectively, annealed at 1100 F. for 1 hour in aconventional heattreating furnace and inspected for blisters. The coatedgroup was found to be blister-free whereas 80 percent of the untreatedgroup was rejected as having 1 or more blisters per panel surface.

Example 2 A lot of brazing sheet panels 14%" x 20" and 0.040 inch thickcomprised of a core of aluminum-7.5% silicon alloy and cladding sheetsnominally composed of aluminum and 1.2 percent manganese was dividedinto two groups. The first group was electrolytically plated withnickel, 0.1 to 0.2 mil thick, over a copper strike, whereas the secondwas not so treated. Both groups were subjected to a degassing operationby heating at 1000 F. for 1 minute. The metal coating waselectrolytically stripped in a sulfuric acid bath. The two groups werethen pressure-formed into evaporator sheet at 1100 F., after which theywere inspected by X-ray to determine the presence of "rivulets or darkspots, which are considered to be gas-filled voids. The treated groupwas found to be free from any such indications whereas the untreated wasall rejected because of such indications.

Example 3 A lot of commercial purity aluminum (99.00 percent minimum)sheet, 0.091 inch thick, was divided into two groups, the first beingelectrolytically plated with chromium (directly), 0.1 to 0.2 mil thick.Both groups were subjected to degassing by heating at 940 F. for 15minutes, after which the coated group was electrolytically stripped in asulfuric acid electrolyte. The material was then subjected to a blisteranneal test, in which occluded gas is determined by heating the specimenunder vacuum at 1100 F. and observing the formation of blisters. Thetreated specimens were free from blister formation whereas the untreatedwere highly blistered.

Having thus dsecribed the invention, we claim:

1. The method of substantially reducing the gas content of aluminumarticles comprising: coating the surface of an aluminum articlecontaining gas with at least one metal selected from the groupconsisting of nickel and chromium; and thereafter heating said coatedarticle in a gaseous atmosphere at a temperature above 750 F.,

but below the temperature at which any substantial amount of fusionoccurs, for a length of time sufficient to diffuse occluded gas into theatmosphere around said article.

2. The method in accordance with claim 1 wherein said atmosphere is air.

3. The method in accordance with claim 1 wherein said atmospherecontains from 1.5 to 30 grains of water per cubic foot.

4. The method in accordance with claim 1 wherein the metal coating isnickel.

5. The method in accordance with claim 1 wherein the metal coating ischromium.

6. The method of substantially reducing the gas con tent of aluminumarticles comprising: coating the surface of an aluminum articlecontaining gas with at least one metal selected from the groupconsisting of nickel and chromium; heating said coated article in agaseous atmosphere at a temperature above 750 F., but below thetemperature at which any substantial amount of fusion occurs, for alength of time sufiicient to dilfuse occluded gas into the atmospherearound said article and thereafter stripping said coating from saidarticle.

7. The method of substantially reducing the gas content and voids inaluminum articles comprising: coating an aluminum article containing gasand voids with at least one metal selected from the group consisting ofnickel and chromium, heating said article in a gaseous atmosphere at atemperature above 750 F., but below the temperature at which anysubstantial amount of fusion occurs, for a length of time suflicient todiffuse occluded gas into the atmosphere around said article; andthereafter working said article to heal any voids therein.

8. The method in accordance with claim 7 wherein said atmosphere is anair containing 1.5 to 30 grains of moisture per cubic foot.

References Cited in the file of this patent UNITED STATES PATENTSWasserman Sept. 4, 1956 Milliken May 5, 1959 OTHER REFERENCES

1. THE METHOD OF SUBSTANTIALLY REDUCING THE GAS CONTENT OF ALUMINUMARTICLES COMPRISING: COATING THE SURFACE OF AN ALUMINUM ARTICLECONTAINING GAS WITH AT LEAST ONE METAL SELECTED FROM THE GROUPCONSISTING OF NICKEL AND CHROMIUM, AND THEREAFTER HEATING SAID COATEDARTICLE IN A GASEOUS ATMOSPHERE AT A TEMPERATURE ABOVE 750*F., BUT BELOWTHE TEMPERATURE AT WHICH ANY SUBSTANTIAL AMOUNT OF FUSION OCCURS, FOR ALENGTH OF TIME SUFFICIENT TO DIFFUSE OCCLUDED GAS INTO THE ATMOSPHEREAROUND SAID ARTICLE.